Inflatable packers or bridge plugs have long been used in wellbore operations. An inflatable packer typically comprises a tubular base and a surrounding, inflatable elastomeric bladder or sleeve. Fluid passages within the tubular body allow fluids to contact the inflatable bladder and expand the bladder radially outwardly to effect sealing engagement with a borehole or well casing.
Since the elastomeric bladder is subjected to both expansion pressure and abrasion or cutting forces, it has been common to surround the exterior surface of the bladder with a plurality of peripherally overlapping, resilient reinforcing slats or ribs. There is generally sufficient overlap of such slats that upon expansion of the inflatable bladder, the slats remain as a surrounding armor protecting the bladder from abrasion and cuts while also preventing extrusion of the bladder elastomer between the slats in a localized area.
Because the slats cannot effect the sealing of the packer against a wellbore or casing, at least some portions of the reinforcing slats are surrounded by and may be bonded to an outer annular elastomeric cover or packing element which, upon expansion of the inflatable packer, comes into pressure sealing engagement with the wellbore or casing.
The outer sealing cover generally comprises either a single or a plurality of annular circumferential elastomeric pieces located on the outer surface of the reinforcing slats. When a single elastomeric piece is employed it may cover only a portion of the longitudinal length of the slats or, alternatively, it may cover the entire outer surface of the slats. Such single piece covers generally have a uniform thickness along their length, the thickness generally being substantial. Such arrangements are described in U.S. Pat. Nos. 3,837,947, 4,832,120 and 5,143,154.
One difficulty with uniformly thick covers is that once a tear or crack develops in the cover, it propagates rapidly over the cover, ultimately resulting in failure of the cover to maintain a seal after more than its initial cycle of use. The failure due to propagation of cracks or cuts over the cover is not ameliorated in multiple piece covers. Multiple piece covers are similarly of a single thickness which does nothing to arrest or retard the propagation of cracks or cuts within the body of a cover piece.
Bonding of the cover to the slats is desirable particularly in a retrievable packer. However, the bonding of the cover to the slats creates yet another source of stress on the cover elastomer as the packer is inflated. Cracks or tears in the cover can result merely as a consequence of inflation and stresses created by the bonding of the cover to the underlying slats which are experiencing flexure and separation.
An additional difficulty with uniform thickness outer covers is that after a typical inflation/deflation cycle of the packer, the exposed blunt ends of the cover can easily be caught in a subsequent running or retrieval movement of the packer resulting in a tearing or pulling off of the cover.